Processes for the manufacture of plastic products are generally accompanied by gas generation in the polymer melt. These gases cause significant problems in both processing and the quality of the final product through the generation of voids, bubbles, and/or odor. In many cases, the voids and bubbles reduce the strength of the plastic product, and, at the very least, have a detrimental effect on its appearance. Further, plastic food containers with any trace of chemical odor are not acceptable.
Whereas the removal of volatiles during polymer manufacture is an indispensable operation, devolatilization during subsequent processing stages is an additional, often very expensive, task. However, residual amounts of unreacted monomer(s), low molecular weight reaction products, solvents and other diluents, processing aids, decomposition products, and entrapped air or inert gases have to be removed. Flash vaporization, thin film vaporization, and screw devolatilization are the basic methods used to remove these impurities, and various screw designs, vacuum systems, additives, and steam or water injection are used to enhance these methods.
A process combining water injection with vacuum devolatilization is particularly effective for removing impurities having a high diffusion velocity in the resin melt.
It is another matter, however, if the impurities have a very low diffusion velocity in the resin melt. In this case, the thickness of the molten resin does not allow the impurities to leave the resin even during a long residence time. For example, reduction of the limonene content in high density polyethylene recycle is not successful in such a process. Increasing melt temperature, the number of vacuum ports, and adding more water only results in foaming with an insignificant reduction in limonene content. A similar result occurs in a like effort to eliminate odor in polypropylene.